Polyethylene resin foam-molded articles are utilized in various uses as a cushioning packaging material or a thermally insulating material, because of their excellent flexibility and thermal insulation property.
For producing the polyethylene resin foam-molded article, an in-mold foam-molding method is known in which polyethylene resin particles are previously foam-molded (bead-foaming) with a foaming agent such as butane gas, the foamed particles are filled in a mold, and the particles are heat-fused by introducing a heat medium such as steam. In the bead-foaming of the polyethylene resin, a cross-linked polyethylene has been often used because a foamed body having a high expansion ratio and an excellent heat-resistance can be obtained therefrom. Even by using a non-cross-linked polyethylene resin having a good recyclability, it is proposed to produce a molded article having a good moldability (see Patent Documents 1 and 2).
Patent Documents 3 and 4 disclose inventions that a linear low-density polyethylene resin is subjected to a heat-treatment (annealing) to obtain a polyethylene resin having two endothermic peaks on a melting curve obtained by a differential scanning calorimetry (DSC), the peak difference thereof being set at a certain value or more, and that when the obtained polyethylene resin is used as a base resin, a mold-processing range of the obtained polyethylene resin foamed particles can be broadened.
The linear low-density polyethylene is a copolymer of ethylene with an α-olefin, and the larger the amount of the α-olefin which is a comonomer, the lower the melting temperature of the copolymer, and thus it is easily forms crystals having a low degree of crystallinity. For that reason, in order to broaden the peak difference on the melting curve by the heat-treatment, resins having a wide distribution of the comonomer amount are required.
According to Patent Document 4, it can be considered that the distribution of the comonomer amount is broadened by mixing polyethylene resins having a density different from each other.
On the other hand, Patent Document 3 discloses, as foamed particles using a polyethylene resin having a high rigidity, foamed particles using, as a base resin, a blended resin of a high-density polyethylene having a density of 0.940 g/cm3 or more and a melt index (which may also be hereinafter referred to as “MI”) of 0.01 to 0.5 g/10 minutes, and a linear low-density polyethylene resin having a density of 0.920 g/cm3 or more and 0.940 g/cm3 or less and an MI of 0.1 to 10 g/10 minutes. According to Patent Document 3, however, it is necessary to blend a large amount of the high-density polyethylene having a low MI and poor fluidity, and thus there is concern over the limitation of an amount of a resin processed and the decreased productivity due to the increased energy necessary for kneading and the increased pressure of an extruder, when resin particles, which are a starting material of foamed particles, are granulated.
Recently, the uniformity of a polymerization is increased with developments of polymerization catalysts, and it has been difficult to obtain a polyethylene resin having a broad comonomer amount distribution. In addition, when a polyethylene resin having a small amount of comonomers is used in order to improve a degree of crystallinity of a resin, a comonomer distribution in the resin is narrowed by decrease of an absolute quantity of the comonomers, and a peak difference on a DSC curve, obtained on a heat-treatment, is also narrowed. Thus, when it is intended to use a polyethylene resin having a high degree of crystallinity and a high rigidity to obtain foamed particles in order to improve a mechanical strength or to decrease a weight of a foam, the moldability is insufficient such that an open cell ratio in an in-mold foam-molded article is increased when a processing temperature is raised, and there is a defect of a narrow mold-processing range. There are some cases, accordingly, where the method in which the peak difference on the DSC curve is broadened in order to broaden the mold-processing range, as in Patent Documents 3 and 4, cannot be applied.
It has also become clear that foamed particles in which a high-density polyethylene is blended, as in Patent Document 3, cause some cases where the foamed particles are shrunk or the mold-processing range is narrowed, comparted to foamed particles having two endothermic peaks on a melting curve obtained by the differential scanning calorimetry and having a peak difference of a certain value or more.
In order to improve a mold-processability of foamed particles, Patent Documents 5 and 6 also disclose foamed particles using, as a base resin, a mixed resin of two or more kinds of polyethylene resins. According to these techniques, however, it is necessary to mix with a low-density polyethylene, produced by a high pressure method, having a low melting point and a low rigidity, and thus there is concern over the reduced heat-resistance and the reduced mechanical properties.
On the other hand, volatile organic foaming agents, as in Patent Documents 1 to 4, have hitherto been used as a foaming agent used in this field because foamed particles having a high expansion ratio can be obtained. With increased interest in environmental issues, however, inorganic gases such as carbon dioxide gas have recently been used as the foaming agent (see Patent Documents 7 and 8). For that reason, even if the same conventionally used resin is used, foamability and mold-processability may sometimes be different. For example, when the foaming agent is used, a ratio of open cells (hereinafter referred to as “open cell ratio”) of a molded article is increased when a heating temperature is raised upon the mold-processing (in-mold foam-molding), and the appearance and physical properties of the molded article may sometimes be deteriorated.